An on chip bias current is used to operate internal circuit components. The on chip bias current is generated by dividing an internal voltage of the chip with respective internal resistance. The voltage used for the on chip bias current can be constant over temperature to generate a constant current, or it can be proportional to temperature to generate a PTAT current.
FIG. 1 illustrates a circuit internal to a chip 102 that generates a constant current 106. In FIG. 1, an internal voltage (Vbg) which is independent of temperature can be generated using a bandgap circuit. Then, this voltage is applied across an internal resistor R0, thus generating a constant current 104 (e.g., Vbg/R0) which flows through the resistor R0, a transistor M1 and a transistor M2. The constant current 106 is proportional to the constant current 104 where the proportion between the two currents is determined by a current mirror consisting of the transistor M2 and a transistor M3. With the ratio of M3 to M2 being a constant K, then the constant current 106 become K*Vbg/R0. The equation illustrates that the constant current 106 can be process dependent and temperature dependent due to the resistor R0 which possesses such characteristics.
FIG. 2 illustrates a typical circuit internal to a chip 202 that generates a PTAT current 206. It is well known in the art that the difference in the base to emitter voltages of the two PNP BJTs (e.g., P1 and P2) is proportional to the temperature of the circuit. That is, V2=V1=Ve1, and a PTAT current 204 crossing a resistor R1 is obtained by (Ve1−Ve2)/R1=(Vbe2−Vbe1)/R1. The PTAT current 206 is proportional to the PTAT current 204 where the proportionality is determined by the current mirror consisting of a transistor T1, a transistor T2 and a transistor T5. In this circuit, the resistor R1 is process and/or temperature dependant.
A typical internal resistor used for the current generation may have resistance that depends on process variations as well as the operating conditions such as temperature and voltage across the resistor. The variations in resistance can become as much as ±30%. An external resistor may be used in place of the internal resistor to decrease the effect of the internal resistor on the bias current. However, such scheme may add extra pins and/or components to the circuits, thus adding to the cost and increasing the size of the circuit. The cost may become even greater if more than one current needs to be generated.